1. Field of the Invention
The present invention relates to a V-belt continuously variable transmission (CVT) for transmitting a driving force of an engine to a driving wheel, and a straddle-type vehicle having such a V-belt CVT disposed below a seat of the vehicle.
2. Description of the Related Art
Recently, there is an increasing demand for straddle-type vehicles called “ATVs (all-terrain vehicles).”
Also, there have been proposed various types of ATVs in which four wheels are provided, each wheel having a wide and low-pressure balloon tire or the like on the left and right sides of the front and rear sides of a body frame, the upper portion of the body frame is provided with steering handlebars, a fuel tank, and a straddle-type seat, arranged sequentially from the front wheel side to the rear wheel side, and a V-belt CVT disposed below the seat and for transmitting a driving force of an engine to driving wheels (see JP-A-2004-156657, for example).
The above-described V-belt CVT includes a primary sheave disposed on a primary sheave shaft to which the driving force of the engine is input, and having a movable sheave half and a fixed sheave half to form a V-groove for receiving a belt; a secondary sheave disposed on a secondary sheave shaft from which a driving force for the driving wheels is output, and having a movable sheave half and a fixed sheave half to form a V-groove for receiving a belt; an endless V-belt received in the respective V-grooves of the primary sheave and the secondary sheave to transmit a rotational driving force between both the sheaves; and a sheave drive mechanism for displacing the movable sheave half of the primary sheave in the axial direction to control the speed change ratio through resulting variations in respective groove widths of the primary sheave and the secondary sheave.
A previous type of sheave drive mechanism, in general, was the so-called centrifugal type which utilized centrifugal force produced according to the engine speed to displace a movable sheave half in the axial direction.
However, when the road on which the vehicle is running turns from a flat road to a climbing road, for example, the sheave drive mechanism of the centrifugal type does not perform a groove width control operation (specifically, a speed change operation) responsively according to changes in the road condition, irrespective of the intention of the rider, until the engine speed has actually decreased because of the increased load from the road. That is, the sheave drive mechanism has a problem of delayed response.
In order to solve the problem of delayed response, another type of V-belt CVT for use in scooter-type motorcycles has been proposed, which includes an electric motor, a control device for controlling the electric motor, a sheave drive mechanism for transmitting a driving force of the electric motor to the movable sheave half of the primary sheave to adjust the respective groove widths of the primary sheave and the secondary sheave, and a rotational speed sensor for detecting rotation of the primary sheave or the secondary sheave to inform the control device of the detected rotation so that the control device can perform a control according to the engine operating condition (see JP-B-2852994, for example).
In general, the scooter-type motorcycle is provided with an integral power unit including an engine and a V-belt CVT attached to a side of a crankcase of the engine. In such a scooter-type motorcycle, the power unit may be swingably suspended from a body frame to function as a swing arm for swingably supporting a rear wheel.
For V-belt CVTs for use in ATVs, there is a need to electrically control the respective groove widths of the primary sheave and the secondary sheave in order to improve response in the speed change process.
However, since ATVs are significantly different in structure from scooter-type motorcycles, the installation method of an electric motor and a rotational speed sensor used in V-belt CVTs of scooter-type motorcycles cannot be used in ATVs.
That is, the engine and the V-belt CVT must be made compact in ATVs when a footrest for the rider is disposed on the outer side of the crankcase and the V-belt CVT. Thus, it is not easy to ensure an installation space for the electric motor and the rotational speed sensor, even on the outer side of the vehicle body. This is because the rotational speed sensor would project greatly to the outer side of the vehicle body and hence prevent the rider from putting his/her foot in place, thereby interfering with the usability of the ATV.
In the case where the rotational speed sensor is located around the outer periphery of the primary sheave or the secondary sheave, a transmission casing is increased in size and hence the V-belt CVT cannot be made compact.
In addition, in the case where the rotational speed sensor is installed near a heat producing part such as a cylinder block of the engine, the service life of the rotational speed sensor and/or the measurement accuracy of the sensor may be reduced and therefore the original performance cannot be maintained.